Cancer remains one of the leading causes of death in the United States. Clinically, a broad variety of medical approaches, including surgery, radiation therapy and chemotherapeutic drug therapy are currently being used in the treatment of human cancer (see the textbook CANCER: Principles & Practice of Oncology, 2d Edition, De Vita et al., eds., J. B. Lippincott Company, Philadelphia, Pa., 1985). However, it is recognized that such approaches continue to be limited by an inability to predict the likelihood of metastasis and tumor recurrence or the most efficacious treatment regime for minimizing the occurrence of these negative outcomes.
Human cancer cells typically contain somatically altered nucleic acids, characterized by mutation, amplification, or deletion of critical genes. In addition, the nucleic acids from human cancer cells often display somatic changes in DNA methylation (36, 37, 38). However, a precise role for, and the significance of, abnormal DNA methylation in human tumorigenesis has not been well established.
Loss of gene function is cancer can occur by both genetic and epigenetic mechanisms. The best-defined epigenetic alteration of cancer genes involves DNA methylation of clustered CpG dinucleotides, or CpG islands, in promoter regions associated with the transcriptional inactivation of the affected genes. CpG islands are short sequences rich in the CpG dinucleotide, and can be found in the 5′ region of about half of all human genes. Methylation of cytosine within 5′ CGIs is associated with loss of gene expression and has been seen in a number of physiological conditions, including X chromosome inactivation and genomic imprinting. Aberrant methylation of CpG islands has been detected in genetic diseases such as the fragile-X syndrome, in aging cells and in neoplasia. About half of the tumor suppressor genes which have been shown to be mutated in the germline of patients with familial cancer syndromes have also been shown to be aberrantly methylated in some proportion of sporadic cancers, including Rb, VHL, p16, hMLH1, and BRCA1 (reviewed in Baylin, et al, Adv. Cancer Res. 72:141-196 1998). Methylation of tumor suppressor genes in cancer is usually associated with (1) lack of gene transcription and (2) absence of coding region mutation. Thus CpG island methylation can serve as an alternative mechanism of gene inactivation in cancer.
Cancer treatments, in general, have a higher rate of success if the cancer is diagnosed early, and treatment is started earlier in the disease process. A relationship between improved prognosis and stage of disease at diagnosis can be seen across a majority of cancers. Identification of the earliest changes in cells associated with cancer is thus a major focus in molecular cancer research. Diagnostic approaches based on identification of these changes in specific genes may allow implementation of early detection strategies and novel therapeutic approaches. Targeting these early changes will lead to more effective cancer treatment.
Despite advances in targeted therapy, surgery with curative intent remains the best therapeutic option for lung cancer patients with the earliest stages of disease. Ensuring in these patients that no occult metastatic cells have disseminated outside the area of curative resection is critical, because early spread of tumor cells is a leading cause of relapse (1-3). Despite the curative aim of early surgery, approximately 30%-40% of lung cancer patients with discrete lesions and histologically proven cancer negative lymph nodes (stage 1:T1-2N0) still die of recurrent disease (4-6). Further, many of these recurrences are systemic, underscoring the probability that these patients had metastatic disease that was undetectable, and beyond the margins of surgical resection.
Accordingly, there is a need in the art for improved methods of detection of proliferative disease, and in particular, for improved methods of detection of metastatic cancer that is undetectable by current methodologies.